To load most 35 mm cameras, a film cartridge is inserted in a loading chamber of the camera and the forward end portion of a film leader protruding from a light-trapping slit in a housing or shell of the cartridge is placed over a take-up spool in the camera. In some cameras, the forward end portion of the film leader is manually attached to the take-up spool before a rear door of the camera is closed. Then, a film loading or winding operation is performed, for example, by manually pivoting a winding lever for the take-up spool and manually depressing a shutter release button several times. This is done until the entire leader is unwound from a supply spool inside the housing and is wound onto the take-up spool, and the first-available film frame is positioned for exposure. In other cameras, the forward end portion of the film leader is automatically secured to the take-up spool at the beginning of the film loading operation. As the take-up spool is rotated via a motor drive, for example, one or more circumferential teeth of the spool engage the forward end portion of the film leader at its perforations to wind the leader onto the spool and position the first-available film frame for exposure. A spring-like deflector or other suitable means may be provided on the rear door of the camera for pressing the film leader against the take-up spool to facilitate engagement of the forward end portion of the leader by the circumferential teeth of the spool.
A problem that exists in some 35 mm cameras is that even though the photographer believes the forward end portion of the film leader is secured to the take-up spool, the forward end portion may fail to be engaged with the spool or may become disengaged from the spool during the film loading operation. As a result, the film leader will not be wound onto the take-up spool and the first-available film frame cannot be positioned for exposure. However, since the rear door of the camera is closed, the photographer may not be aware of the malfunction.